Of the conventional recording media, a removable optical disk for use with a disk recording and/or playback apparatus is rotatable on and with a turntable included in a disk rotation driving mechanism when it is set in the apparatus and clamped to the turntable. Being rotated, the optical disk clamped at the center thereof to the turntable will easily incur a so-called axial runout. Namely, the optical disk will be oscillated vertically about its clamped center while being rotated.
The axial runout, if any, of the optical disk set in the disk recording and/or playback apparatus and being rotated by the disk rotation driving mechanism is likely to cause primarily a focusing error when recording or reproducing information.
Therefore, the clamping to the turntable in the recording and/or playback apparatus should desirably be done with the removable optical disk being mounted horizontally on the turntable without being tilted. However, it is difficult to mount and clamp the optical disk precisely horizontally on the turntable.
On this account, in the recording and/or playback apparatus using a removable optical disk as a recording medium, a focusing servo mechanism is provided in the recording and/or playback apparatus to address an axial runout which will possibly arise to some extent while the optical disk is being rotated by the disk rotation driving mechanism. For example, a DVD (digital versatile disk) recording and/or playback apparatus includes a focusing servo mechanism designed to prevent any focusing error from occurring even when an axial runout of about ±300 μm takes place in relation to the plane of rotation.
Recently, it has been proposed to use evanescent light in an apparatus that records or reads predetermined information by projecting a light beam to an optical recording medium such as an optical disk, in order to enable a high-density recording or reproduction beyond the diffraction-limited.
For writing or reading information to or from an optical recording medium with evanescent light, there has been proposed to use an SIL (solid immersion lens) for producing the evanescent light.
For writing or reading information to or from an optical recording medium with the use of evanescent light, the optical system to collect or condense light beam for projection onto the optical recording medium should use a two-group lens composed of an SIL and aspheric lens and having a numerical aperture (NA) of more than one (1) and have a light-emitting surface provided a distance which is smaller than a half of the wavelength of the light beam incident upon the SIL apart from the information recording surface of the optical recording medium. For example, when the wavelength λ of the light beam is 400 nm, the distance is less than 200 nm.
For quality recording or reproduction of information, the distance between the light-emitting surface of the light-condensing optical system and the information recording surface of the optical recording medium should be kept constant. On this account, there has been proposed a technique of controlling the position of the light-condensing optical system in relation to the light recording medium by controlling the actuator of the optical head according to a variation of return-light amount from the optical recording medium as an error signal, as disclosed in the Japanese Patent Application Laid Open No. 2001-76358.
As mentioned above, to enable quality recording and reproduction with the evanescent light, it is necessary to have the optical head follow the information recording surface of the optical recording medium while controlling the position of the light-emitting surface of the light-condensing optical system and that of the information recording surface of the recording medium at extremely short intervals of the order of nano-meters.
It is difficult to rotate a removable optical recording medium being mounted precisely horizontally on the disk rotation driving mechanism of a disk recording and/or playback apparatus and prevent an axial runout of the recording medium being rotated. Specifically, it is extremely difficult to limit the axial runout relative to the plane of rotation of the recording medium to less than ±300 μm. Thus, in a recording and/or playback apparatus using evanescent light of 400 nm in wavelength, it is extremely difficult to limit the gap between the light-emitting surface of the light-condensing optical system and the information recording surface of the optical recording medium to less than 200 nm for writing or reading information to or from the optical recording medium.
On the assumption that in a recording and/or playback apparatus using evanescent light of 400 nm in wavelength, the gap between the light-emitting surface of the light-condensing optical system and the information recording surface of the optical recording medium is 100 nm and a gap error of ±1% is allowable, it is necessary to control gap error to less than ±1 nm. In such an apparatus, if an axial runout of about 300 μm which is allowable in DVDs etc. takes place, focusing servo control needs a DC gain of more than 100 dB. Designing a stable control system while maintaining a DC gain of more than 100 dB is extremely difficult.
In a recording and/or playback apparatus in which an optical recording medium is removably set in a rotation drive, it is very difficult to limit the axial runout of the optical recording medium being rotated to less than ±10 μm for example in advance.
Making the allowable gap error higher than the current requirement is also possible but it will degrade read RF signal or the like and will thus lead to impossibility of quality recording or reproduction of information.
Also, the recording and/or playback apparatus to write or read information to or from an optical recording medium using the aforementioned evanescent light will not be able to use any recording medium having widely been prevailing as a large information source, such as CD (compact disk), DVD (digital versatile disk) or the like.
The recording and/or playback apparatus to write or read information by evanescent light and recording and/or playback apparatus to write or read information by condensing laser light from a laser source and projecting it onto the information recording surface of a disk such as CD, DVD or the like are different from each other in configuration of the optical system and optical head thereof. Therefore, to implement interchangeability between the optical recording medium for recording and/reading information by evanescent light and the CD and DVD, it is necessary to design a recording and/or playback apparatus including optical systems for the evanescent light-type recording medium and the CD and DVD, respectively, can selectively be used for each of such media. However, there remain problems such as addressing the incorrect selection of the optical system for an optical recording medium to be used, larger size of the apparatus, increased costs due to the larger size, etc.